Replaced HTMLIdentifier usage with a factory function which will try to match the string with a map of all known static strings.

As the HTMLNames strings are now static and hence threadsafe, the HTMLIdentifier wrapper used for thread safe string usage by the parser thread is no longer needed.

Sourced from discussions in https://codereview.chromium.org/74513003/

R=abarth@chromium.org,eseidel@chromium.org
BUG=277886

Review URL: https://codereview.chromium.org/110843004

git-svn-id: svn://svn.chromium.org/blink/trunk@163806 bbb929c8-8fbe-4397-9dbb-9b2b20218538